A carbon fiber reinforced composite material, which is a composite of carbon fibers and a matrix resin, has been used in a wide range of fields including automobiles, aircrafts, electrical and electronic equipment, optical instruments, sports equipment, and building materials because it has excellent lightness, mechanical property, conductivity, dimensional stability, and the like.
A number of methods of forming a carbon fiber composite material are known, and examples of the method for obtaining a substrate used in the formation include a method involving processing carbon fibers using an aqueous process represented by a wet paper making process. For example, chopped fibers are processed into paper or a nonwoven fabric by dispersing them in an aqueous medium, and then a composite material substrate is obtained using various resins as a base material. For example, an electrode substrate for a fuel cell is produced by this paper making process.
In a wet paper making process, paper is made by dispersing chopped fibers in an aqueous dispersion medium. For improving the quality of paper to be obtained, carbon fibers' convergence property and dispersibility in the aqueous dispersion medium are required. Convergence property is important for equalization of fiber length upon cutting and workability in feeding of a chopped fiber. Dispersibility is a property for fiber bundles to be dispersed at a single fiber level and directly influences the paper quality. The more excellent the carbon fiber bundle's convergence property and dispersibility, the more the mechanical property and electrical conductive properties of the carbon fiber reinforced composite material improve.
From such a background, in the carbon fiber bundle used in an aqueous process, a particular resin is generally deposited as a sizing agent to improve the convergence property and dispersibility.